A Simple Electrochemical Approach to Heterogeneous Reaction Kinetics

Despite the great importance of reactions involving solids and solutions (dissolution reactions, crystallization reactions, heterogeneous catalysis, adsorption and ion exchange reactions), very few teaching laboratory experiments attempt to inculcate the fundamentals of this type of reaction. A major difference between such heterogeneous reactions and the usual homogeneous reactions in solution is that, in the former, the reactant in solution must reach the solid surface before reaction can take place. As a result, reactions between solids and solutions are often controlled by transport of the reacting solution species to the solid surface.It is not easy to experimentally obtain reproducible mass transport. In the teaching experiment described in this paper, it is shown that reproducible results can be obtained using a solid sample in the form of a rotating disc. However, the principle can also be demonstrated using a fixed sample and a stirred solution. The system studied is the dissolution of a known amount of copper in a solution of iron(III). The reaction time is determined by measuring the potential of the copper, which has been electrodeposited on an inert electrode, as a function of time; the potential increases rapidly when all the copper has been consumed. The reaction rate is shown to be dependent on the stirring of the solution, and is first order with respect to the reactant, iron(III).